This invention relates to a process for producing a silica glass usable for optics, semiconductor industry, electronic industry, physical and chemical applicances, etc.
Silica glass is an inevitably important material for producing semiconductors due to its excellent heat resistance, corrosion resistance and optical properties. Further, the silica glass is also used as optical fibers, photo mask substrates for producing IC, TFT substrates and is broadening it utility more and more.
The silica glass has been produced by a method wherein natural quartz is melted in an electric furnace or by an oxyhydrogen flame, or a method wherein silicon tetrachloride is subjected to high-temperature oxidation and melted in an oxyhydrogen flame or a plasma flame. But these methods have many problems in economy and quality in that a large amount of energy is consumed in the production processes because of necessitating a high temperature of 2000.degree. C. or higher, it is necessary to use a material durable at such a high temperature in the production processes, a highly pure product is difficult to be obtained, and the like.
Recently, there has been noticed a sol-gel process wherein a silica glass is synthesized at low temperatures. According to the sol-gel process, the silica glass is produced by adding water to a silicon alkoxide represented by the formula: EQU Si(OR).sub.4
wherein R is an alkyl group, and/or a silicon alkoxide polycondensate represented by the formula: EQU (RO).sub.3 Si.multidot.[OSi(OR).sub.2 ].sub.n .multidot.OSi(OR).sub.3
wherein R is an alkyl group; and n is zero or an integer of 1 to 8, (if necessary, a pH can be adjusted with an alkali or acid), carrying out hydrolysis to give a silica hydrosol (hereinafter referred to as "silica sol") while adding an solvent such as an alcohol so as to make a uniform system of the silicon alkoxide and water, allowing the silica sol to stand, followed by a temperature rise, to carry out gelation by addition of a gelatinizer, evaporating and drying the resulting gel to give a dry gel, and sintering the dry gel in a suitable atmosphere.
But there are still unsolved problems in the production of silica glass by the sol-gel process. For example, in the course of drying the gel, cracks and fractures are easily produced on the gel. Thus, it is very difficult to produce a large-sized monolithic dry gel without cracks and fractures in good yield.
In order to prevent cracks and fractures of the gel, it is proposed to add formamide at the hydrolysis of silicon alkoxide (Mat. Res. Soc. Symp. Proc., 32, (1984), 47-52) or to add a solvent having a higher boiling point than water such as 1-butanol, 1-pentanol, 1-hexanol, or toluene (Japanese Patent Unexamined Publication No. 61-183129). But the prevention of cracks and fractures are still insufficient. It is also proposed to add a fine powder of silica at the hydrolysis of silicon alkoxide (U.S. Pat. No. 4,681,615). According to this method, cracks and fractures of the gel can be prevented, but fine powder of silica is readily retained not sintered, which results in readily lowering optical properties, particularly light transmittance, of the obtained glass.